1. Technical Field of the Invention
The present invention relates generally to a gas sensor which may be installed in an exhaust system of an internal combustion engine to determine the concentration of O2, an air-fuel ratio, or the concentration of NOx in exhaust emissions, and more particularly to an improved structure of such a type of gas sensor equipped with a cover assembly designed to avoid the breakage of a sensor element arising from splashing with water without sacrificing a response speed of the gas sensor.
2. Background Art
FIG. 19 illustrates a typical example of an oxygen sensor 9 (also called O2 sensor) which is to be installed in an exhaust pipe of an automotive internal combustion engine (not shown) to measure the concentration of oxygen (O2) contained in exhaust gas G as a function of the air-fuel ratio of a mixture charged into the engine for use in controlling the combustion in the engine.
The oxygen sensor 9 includes a gas sensor element 92 and a protective cover assembly 93. The gas sensor element 92 is equipped with a solid electrolyte body made of zirconia and disposed within the protective cover assembly 93. The protective cover assembly 93 is made of metal such as stainless steel and has formed therein gas inlets 933 through which the exhaust gas G is admitted inside the protective cover assembly 933.
The exhaust gas G enters the protective cover assembly 93 at the gas inlets 933 and reaches the gas sensor element 92. The gas sensor element 92 is sensitive to the exhaust gas G to produce a signal as a function of the concentration of oxygen.
During the rest of the engine, the moisture contained in the exhaust gas G may touch on a cooled inner wall of the exhaust pipe cooled so that it is condensed into drops of water. When the temperature of the exhaust gas G is low immediately after the start-up of the engine, the drops of water may be blown away by the exhaust gas G without being evaporated and enter the protective cover assembly 93 together with the exhaust gas G.
The correct measurement of the concentration of oxygen requires keeping the gas sensor element 92 at high temperatures of 400° C. or more, i.e., at an activated state. The adhesion of the drops of water to the surface of the gas sensor element 92 within the protective cover assembly 93 may, thus, cause the gas sensor element 92 to be subjected to thermal stress and broken.
In order to minimize the adhesion of water to the gas sensor element 92, the protective cover assembly 93 is, as clearly illustrated in FIG. 19, made to have a double-wall structure equipped with an inner cover 931 and an outer cover 932 and also have the gas inlets 933 of the inner cover 931 placed in misalignment with those of the outer cover 932 in a direction of flow of the exhaust gas G.
However, when the water drops Ware, as illustrated in FIG. 19, adhered to the outer surface 934 of the outer cover 932, they may move on the outer surface 934 to the gas inlets 933 and enter inside the outer cover 931. The water drops W may further slide on the outer surface 936 of the inner cover 931 or the inner surface 935 of the outer cover 932 to the gas inlets 933 of the inner cover 931 and then enters inside the inner cover 931, so that they stick to the gas sensor element 92, thereby resulting in breakage of the gas sensor element 92.
In order to avoid the above problem, Japanese Patent First Publication No. 8-240559 teaches, as illustrated in FIG. 20, the gas sensor element 92 covered with a water-repellant protective film 94 to resist the adhesion of the water to the gas sensor element 92.
The installation of the protective film 94 on the surface of the gas sensor element 92, however, will result in an increased time required for the exhaust gas G to reach a sensing portion of the gas sensor element 92, which leads to a delay in response of the gas sensor element 9. It also results in an increase in thermal capacity of the gas sensor element 92, thus prolonging the time required to bring the gas sensor element 92 into the activate state.
Japanese Utility Model First Publication No. 4-11461 teaches, as illustrated in FIG. 21, a gas sensor 90 equipped with a protective layer 940 formed on a protective cover 93 to cover gas inlets 933. When the protective layer 940 covers the gas inlets 933 too broadly, it will, like the above publication, will result in an increased time required for the exhaust gas G to reach a gas sensor element 920, which leads to a delay in response of the gas sensor element 9.